Investigations of vibrational coherence in the low-frequency region of ferric heme proteins.

Femtosecond coherence spectroscopy is applied to a series of ferric heme protein samples. The low-frequency vibrational spectra that are revealed show dominant oscillations near 40 cm(-1). MbCN is taken as a typical example of a histidine-ligated, six-coordinate, ferric heme and a comprehensive spectroscopic analysis is carried out. The results of this analysis reveal a new heme photoproduct species, absorbing near 418 nm, which is consistent with the photolysis of the His(93) axial ligand. The photoproduct undergoes subsequent rebinding/recovery with a time constant of approximately 4 ps. The photoproduct lineshapes are consistent with a photolysis quantum yield of 75-100%, although the observation of a relatively strong six-coordinate heme coherence near 252 cm(-1) (assigned to nu(9) in the MbCN Raman spectrum) suggests that the 75% lower limit is much more likely. The phase and amplitude excitation profiles of the low-frequency mode at 40 cm(-1) suggest that this mode is strongly coupled to the MbCN photoproduct species and it is assigned to the doming mode of the transient penta-coordinated material. The absolute phase of the 40 cm(-1) mode is found to be pi/2 on the red side of 418 nm and it jumps to 3pi/2 as excitation is tuned to the blue side of 418 nm. The absolute phase of the 40 cm(-1) signal is not explained by the standard theory for resonant impulsive stimulated Raman scattering. New mechanisms that give a dominant momentum impulse to the resonant wavepacket, rather than a coordinate displacement, are discussed. The possibilities of heme iron atom recoil after photolysis, as well as ultrafast nonradiative decay, are explored as potential ways to generate the strong momentum impulse needed to understand the phase properties of the 40 cm(-1) mode.